1. Field of the Invention
This invention related to a light diffusing film and its manufacture, and a polarizing plate with a light diffusing layer, and a liquid crystal display apparatus which are suitably used for the display of a wordprocessor, computer, television set or the like, for a polarizing plate or an optical lens to be incorporated in a liquid crystal display apparatus, and as for a cover of various meters, and as the window-shield glass of an automobile or of a railway coach.
2. Prior Art
The display apparatus of a computer or the like as described above uses various light diffusing films to improve its visibility.
Of such light diffusing films some depend on surface roughness to assume a light diffusing activity while others depend on a diffusing material contained in a resin film where the difference in refractive indices between the resin and the diffusing material is responsible for the light diffusing activity.
When the light diffusing activity of a given film depends on its surface roughness, the light diffusing activity and the transparency of film vary according to from which angle it is seen, and thus the visibility thereof varies according to from which angle it is seen, which poses a problem. For example, with a display apparatus incorporating such a film, an image on display is quite visible when seen from front, but the same image becomes whitish when the visual angle becomes large.
Such a light diffusing film containing a diffusing material in its resin substrate as described above is disclosed, to take as an example a film applied to a reflection type liquid crystal display apparatus, in Journal of Illumination Research Society MD-96-48 (1996) pp. 277-282.
According to this disclosure, to obtain a reflection type liquid crystal display apparatus with a high contrast, it is important to design the apparatus such that the frontal light diffusing intensity is appropriately adjusted, and the backward light diffusing intensity is strongly suppressed, because such design is advantageous for the realization of a bright display and a high resolution, and further ensures the realization of a birefringent film which will compensate for the optical performance of liquid crystal which, in turn, will be beneficial for widening the effective view angle.
The disclosure further asserts that, to achieve above object, it will be better to use a film where light diffusing occurs as a result of mismatch in refractive indices among film constituents, rather than a film where light diffusing occurs as a result of surface roughness, because with the latter film the backward light diffusing intensity will become so large as to lower the contrast. Namely, the disclosure recommends the use of a resin film which contains a diffusing material within, where light diffusing activity is invoked by the difference in refractive index among film constituents.
The above-cited article in Journal of Illumination Research Society MD-96-48 states, utilizing the light diffusing theory offered by Mie and the theory by Hartel as a concept to support the design of an effective light diffusing film, it is possible to obtain a film with an optimum light diffusing activity, by appropriately adjusting the relative refractive index m between a diffusing material and a resin, and the size parameter xcex1 and particle density parameter Nd of diffusing material, and particularly it is useful for achievement of above purpose to keep the size parameter xcex1 at 10 or more.
The size parameter xcex1 is xcex1=2xcfx80R/xcex, and thus it depends on the diameter R of diffusing material (80  represents the wavelength of light).
Accordingly, if the size parameter xcex1 be 10 or more, inevitably the diffusing material will have a large particle size in association. If a diffusing material with such a large particle diameter consisting, for example, of spherical particles made of a plastics, is allowed to disperse in a transparent polymer (resin), the resultant film will become thick, and thus usable combinations of a diffusing material and resin for molding will be limited, or the film will develop a birefringence when molded by extrusion. These pose a problem when the film is put to practice.
Further, when such a light diffusing film is applied to a liquid crystal display, it must be applied to the outside surface of liquid crystal panel, because, if it were applied to the inside surface of liquid crystal panel, it would interfere with the polarizing activity of the display unit.
The alternative, conventional method whereby reflection of rays incident on the surface of a transparent substrate is prevented includes a method wherein an anti-reflection coat is applied on the surface of a glass or plastics substrate, a method wherein an ultra-thin film made of MgF2 or other metals having a thickness of about 0.1 xcexcm is applied by vapor deposition on a transparent substrate made of glass or the like, a method wherein an ionizing-radiation setting resin is coated on the surface of a plastics acting as a plastics lens, and then another film made of SiO2 or MgF2 is plated by vapor deposition on the former, and a method wherein an ionizing-radiation setting resin is coated and on that hardened coat is applied a film with a low refractive index.
However, for those conventional films to be given a light diffusing activity, only the outermost layer is available, and thus they can not have an anti-reflection layer if a light diffusing layer is prepared on the outermost layer. Hence, if such a light diffusing film is applied onto the surface of panel of a liquid crystal display apparatus, reflection of rays incident on the surface is not sufficiently prevented.
Further, with a conventional reflex type liquid crystal apparatus, it is generally a custom to place polarizing plates on both surfaces of a liquid crystal cell, and to add to one of them a diffuse reflection plate having a roughened surface.
However, when the diffuse reflection plate is placed on the outside surface of liquid crystal cell, a ghost display called parallax develops depending on the thickness of glass of liquid crystal cell, which greatly impairs the visibility of the display.
To prevent the development of parallax, a method has been developed wherein only a single polarizing plate is used, and a metal electrode in the liquid crystal cell is allowed to act also as a light reflection agent. When the metal electrode assumes a light reflecting property by acting as a mirror, the visual angle becomes narrow, and brightness along the normal decreases. When the metal electrode assumes a light diffuse reflection property by having a roughened surface, it becomes difficult to control the orientation of liquid crystal, and the production processes become complicated. This poses a new problem.
The present invention has been performed in consideration of the above-mentioned existing problems, and an object of the invention is to provide: a light diffusing film which is thin, does not impose any restrictions on the selection of diffusing materials and resins, does not develop a birefringence, and is also applied in the interior of display panel, and its manufacture; and a polarizing plate and a display apparatus each with a light diffusing layer attached thereto.
A further object of the present invention is to provide a light diffusing film which further includes an anti-reflection layer to sufficiently prevent the occurrence of reflection rays out of incident rays coming from outside, and its manufacture; and a polarizing plate and a liquid crystal display apparatus each with a light diffusing layer.
The present invention attains the above-mentioned object by means of a light diffusing film which has, on at least one of the surfaces of a transparent film substrate, a light diffusing layer laminated which comprises a light transmissive resin containing a light transmissive diffusing material being different from the light transmissive resin in refractive index, wherein the haze value on the surface of light diffusing layer is three or more, the difference between the haze value along the normal and that along the lines having angles xc2x160xc2x0 apart from the normal is four or less, and Ra representative of the surface roughness is 0.2 xcexcm or less.
The difference xcex94n in refractive index between the light transmissive resin and the light transmissive diffusing material of the light diffusing layer may be chosen so as to satisfy the inequality 0.01xe2x89xa6xcex94nxe2x89xa60.5, and the average particle diameter of light transmissive diffusing material may be chosen so as to satisfy the inequality 0.1 xcexcmxe2x89xa6dxe2x89xa65 xcexcm.
The present invention attains the above-mentioned object by means of a light diffusing film which has, on at least one of the surfaces of a transparent film substrate, a light diffusing layer laminated which comprises a light transmissive resin containing a light transmissive diffusing material, wherein, between the light transmissive diffusing material and the light transmissive resin of the light diffusing layer, or on at least part of that interface, is inserted a layer having a lower refractive index than those of the light transmissive diffusing material and resin.
The layer with a lower refractive index may be formed of air.
The layer with a lower refractive index may be a coat applied on the perimeter of light transmissive diffusing material.
Further, the haze value on the surface of light diffusing layer may be three or more, and the difference between the haze value along the normal to the surface and that along the lines xc2x160xc2x0 apart from the normal may be four or less.
Still further, the transparent film substrate may be made of either triacetate cellulose or polyethylene terephthalate.
The transmissive resin may have an bonding activity or adhesive activity.
Still further, the light transmissive resin may be a ultra violet setting resin, and the light transmissive diffusing material may comprise melamine beads. Still further, the light diffusing layer may have, on at least one of the front and back surfaces, an anti-reflection layer including an optical thin film.
To achieve above object, this invention provides a manufacture of the light diffusing film as above described which comprises the steps of choosing melamine beads as a light transmissive diffusing material and a UV setting resin as a light transmissive resin, applying the liquid light diffusing layer onto a transparent film substrate, and radiating UV rays thereupon to harden the light transmissive resin.
To achieve above object, this invention provides a manufacture of the light diffusing film as above described which comprises the steps of converting the light transmissive diffusing material into beads, coating their perimeter with a material which, while the light transmissive resin is hardening, will become gas or adsorptive to the light transmissive resin, and applying that matter together with the liquid light transmissive resin onto a transparent film substrate to harden thereupon.
Still further, to achieve above object, this invention provides a manufacture of the light diffusing film as above described which comprises the steps of coating the liquid light diffusing layer onto a transparent film substrate, overlaying a molding film thereupon whose surface has been so finely modified as to have a surface roughness of 0.2 xcexcm or less, and peeling off the molding film, after allowing the initially-coated layer to harden.
Still further, to achieve above object, this invention provides a manufacture of the light diffusing film as above described which comprises the steps of applying a polarizing plate with laminated polarizing layers onto at least one of the surfaces of transparent film substrate, and applying a light diffusing layer on the other surface of transparent film substrate.
An anti-reflection layer including an optical thin film may be laminated on either the polarizing layer or the light diffusing layer.
Further, to achieve above object, this invention provides a liquid crystal display apparatus which has a liquid crystal panel, and a polarizing plate with a coat of light diffusing layer applied on the display surface of liquid crystal panel.
The liquid crystal panel may be of reflex type which has, on its rear surface, a reflective member with a mirror-like reflective activity.
This invention is based on a finding that, assumed that a light diffusing film includes a light diffusing layer consisting of a light transmissive resin containing a light transmissive diffusing material with a different refractive index, preparing the light diffusing layer such that the haze value of the surface be three or more, the difference between the haze value along the normal and that along the lines xc2x160xc2x0 apart from the normal be four or less, and the surface roughness Ra be 0.2 xcexcm or less, will make it possible to render the particle size of light transmissive diffusing material sufficiently small, and the display quality of, for example, liquid crystal display surface good to excellent, and to introduce the resulting light diffusing film in the interior of liquid crystal panel if required.
This invention is based on another finding that, when between the light transmissive diffusing material and the light transmissive resin is inserted a layer which has a refractive index lower than those of the former two, the relative refractive index m between the light transmissive diffusing material and the light transmissive resin becomes large, which will make it possible to reduce the necessary add amount of light transmissive diffusing material.